Structural characterization of fish egg vitelline envelope proteins by mass spectrometry

The extracellular coat, or vitelline envelope (VE), of rainbow trout (Oncorhynchus mykiss) eggs consists of three proteins, called VEalpha (M-r similar to52 kDa), VEbeta (M-r similar to48 kDa), and VEgamma (M-r similar to44 kDa). Each of these proteins is related to mammalian egg zona pellucida (ZP) glycoproteins ZP1-3 and possesses an N-terminal signal sequence, a ZP domain, and a protease cleavage site near the C-terminus. VEalpha and VEbeta also have a trefoil domain. All three proteins possess a relatively large number of cysteine residues (VEalpha, 18; VEbeta, 18; VEgamma, 12), of which 8 are present in the ZP domain and 6 are present in the trefoil domain of VEalpha and VEbeta. Here, several types of mass spectrometry were employed, together with gel electrophoresis of chemical and enzymatic digests, to identify intramolecular disulfide linkages, as well as the N- and C-terminal amino acids of VEalpha, VEbeta, and VEgamma. Additionally, these methods were used to characterize two high molecular weight proteins (HMWPs; M-r > 110 kDa) of rainbow trout VEs that are heterodimers of individual VE proteins. These analyses have permitted assignment of disulfide linkages and identification of N- and C-terminal amino acids for the VE proteins and determination of the protein composition of two forms of HMWPs. These experiments provide important structural information about fish egg VE proteins and filaments and about structural relationships between extracellular coat proteins of mammalian and nonmammalian eggs.